It is difficult for anyone to read intent. I don't see a deliberate attempt to injure or punish - just a ill-timed and overly aggressive attempt to throw a block for the BC, which resulted in an illegal block below the waist (not within the 10-2 region). He'll say he was just doing what his coach told him to do - just keep blocking until you hear a whistle and/or everybody stops. Who knows if he heard a whistle? The rest of the action was behind him at that point - which is no excuse - he needs to be aware of where the BC is, and the status of the ball. And he needs to better understand the BBW rules.
But, I wouldn't call this flagrant, unless someone (an official) that was there can positively say that other players heard the whistle and knew the play was over, and were easing up before this guy committed himself to this block. If that is the case, though, then you certainly could - and should - go with flagrant.
This does reinforce the absolute need for a positive whistle on every play. I think the 'fad' is dying, but, for some 10 tears or so, it seemed to be 'cool' to not have a whistle on plays, with the ill-conceived philosophy that whistles aren't needed, because the plays kill themselves. All of that was a BS excuse to avoid inadvertent whistles. Thankfully, the 40-second clock seemed to be the catalyst to stop that practice, as it seems to have waned significantly since then. By RULE we are to sound our whistle when the ball becomes dead. Players leading blocking for a BC in their immediate vicinity should be able to expect that they can commit themselves to an otherwise legal block, if they haven't heard a whistle.
I would hate to think that this illegal block (and an injured opponent?) could have been prevented with a proper and timely whistle.
Robert